1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography as employed in medicine for examining patients. The present invention is specifically directed to a method for manufacturing a carrier tube for the body coil of an MRI apparatus.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully utilized as an imaging method in medicine and in biophysics for more than 15 years. In this examination modality, the subject is disposed in a strong, constant magnetic field. The nuclear spins of the atoms in the subject that were previously irregularly oriented are aligned as a result. Radio-frequency energy can then excite these xe2x80x9corderedxe2x80x9d nuclear spins to a specific oscillation. This oscillation generates the actual measured signal in MRT that is picked up with suitable reception coils. The examination subject can be spatially encoded in all three spatial directions by utilizing non-uniform magnetic fields generated by gradient coils. The method allows a free selection of the slice to be images, allowing tomograms of the human body to be acquired in all directions. As a tomographic imaging method in medical diagnostics, MRT is distinguished first and foremost by a versatile contrast capability as a xe2x80x9cnon-invasivexe2x80x9d examination modality. MRT currently employs applications with high gradient power that enable an excellent image quality with measuring times on the order of magnitude of seconds and minutes.
The constant technological improvement of the components of MRT devices and the introduction of fast imaging sequences have created an increasing number of medical application in medicine. Real-time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are a few examples.
FIG. 9 shows a schematic section through a conventional MRT apparatus. The section shows further components of the interior that is surrounded by the basic field magnet 1. The basic field magnet 1 contains superconducting magnet coils that are situated in liquid helium and is surrounded by a magnet envelope 12 in the form of a two-shell vessel. The cryo-head 15 that is attached to the magnet envelope 12 at the outside is responsible for keeping the temperature constant. The gradient coil 2 is concentrically suspended via carrying elements 7 in the interior surrounded by the magnet envelope 12 (also called magnet vessel). A carrying tube with the radio-frequency antenna applied thereon is likewise concentrically introduced in the interior of the gradient coil 2. The carrying tube and RF antenna are referred to below as an RF resonator or as a xe2x80x9cbody coilxe2x80x9d 13. The gradient coil 2 and the body coil 13 thus represent two cylinders inserted into one another with a radial spacing therebetweenxe2x80x94in the form of an air gapxe2x80x94amounting only to about 3 cm. The RF antenna converts RF pulses emitted by a power transmitter into a magnetic alternating field for exciting the atomic nuclei of the patient 18, and subsequently converts the alternating field emanating from the precessing nuclear moment into a voltage supplied to the reception branch. The upper part of the body coil 13 is mechanically connected to the magnet envelope 12 via a cladding 29 that is funnel-shaped. Tongues 30 (see FIG. 10) are mounted at the lower part of the body coil 13, the body coil 13 being mechanically connected via these tongues 30 to the lower part of the magnet envelope 12 via a cladding 29 as well as with carrying elements 7. The tongues 30 as well as the body coil 13 are mechanically connected to bed rails 33. Under certain circumstances, the tongues 30 are considered as belonging to the body coil 13. The patient 18 on a patient bed 19 is moved into the opening in the interior of the system via glide rails 17. The patient bed is disposed on a vertically adjustable carrying frame 16.
The gradient coil 2 is likewise composed of a carrying tube 6 having an exterior on which three windings (coils) are disposed that each generate a gradient that is proportional to the current supplied to the coil. The three gradients are perpendicular to one another. A radio-frequency shield (RF shield) 20 that shields the coils from the radio-frequency field of the RF antenna is applied on the inside of the carrying tube 6. As shown in FIG. 11, the gradient coil 2 has an x-coil 3, a y-coil 4 and a z-coil 5 that are respectively wound around the carrying tube 6 and thus respectively generate gradient fields in the directions of the Cartesian coordinates x, y and z. Each of these coils is equipped with its own power supply in order to generate independent current pulses with the correct amplitude and at the correct time in conformity with the sequence programmed in the pulse sequence controller. The required currents lie at approximately 250 A. Since the gradient switching times should be as short as possible, current rise rates on the order of magnitude of 250 kA/s are required. In an extremely strong magnetic field as is generated by the basic field magnet 1 (typically between 0.22 and 1.5 Tesla), such switching events involve strong mechanical oscillations due to the Lorentz forces that thereby occur, these mechanical oscillations leading to considerable noise.
The following demands are made of the body coil 13 of an MRT apparatus:
For space reasons, a tube wall thickness of only up to 10 mm can be accepted. The material of the body coil should comprise an optimally low power absorption of RF power, i.e. must be electrically non-conductive. The body coil must be MR-compatible, i.e. non-imaging in the sense of magnetic resonance (for example, it dare not contain any water). Since the body coil is supposed to carry the patient bed with patient, the body coil must comprise high mechanical shape stability. In order to shield the noise generated mainly by the gradient coil as well as possible, to body coil should be optimally long without comprising interruptions. For design-oriented reasons, however, the funnel-shaped widened portion (cladding 29) of the patient tunnel should also begin as far inside as possible, which leads to a very short body coil and does not meet the noise-related demands.
In conventional solutions, short cylindrical Gfk tubes of epoxy resin are employed for the body coil, the functional elements of the RF antenna being applied thereon in the form of planar copper conductors. For manufacturing such tubes, a rotating arbor is wrapped with resin-saturated fiberglass rovings and is cured (possibly upon application of heat). This solution involves compromises that have a significant disadvantage with respect to one of the two aspects of noise or design: Although the body coil is short, the funnel-shaped widened portion is not a part of the body coil but is composed of a separate plastic part (cladding 29). This is inadequate for meeting the noise-reducing demands since it lacks the necessary mass and rigidity. Second, the interface between the body coil and the funnel-shaped cladding represents an acoustic weak point.
It is an object of the present invention to optimize the noise and design properties as well as the electromechanical stability of a magnetic resonance tomography apparatus.
This object is inventively achieved in a magnetic resonance tomography apparatus having a basic field magnet surrounded by a magnet envelope that surrounds and limits an interior space, with a gradient coil system disposed in this interior space, and a body coil having an RF antenna and a carrying tube disposed in the gradient coil system as an inner encapsulation cylinder, and wherein the magnet envelope and the gradient coil system are optically as well as acoustically closed by the body coil and a diaphragm at the end faces and in the interior. The body coil is inventively manufactured by a vacuum casting process or a vacuum die-casting process.
A body coil manufactured in this way yields a great number of advantages.
First, the casting technique allows significantly more degrees of freedom as to the shaping.
In order to meet optimum noise-related requirements, for example, the body coil can be cast such that has an overall length that is greater than the gradient coil lying behind it.
In order to also meet optimum design requirements, the body coil can be cast such that it is widened with a funnel-shape at one end or at both end faces. This is possible only to a limited extent and with considerable outlay given the conventionally employed winding technique.
When tongues and/or bed rails are provided in the lower region of the body coil, these can be inventively cast with the body coil to form a unit, which leads to a considerably better overall mechanical stability.
Functional elements, which are conventionally glued onto the wound body coil, can likewise be cast with the body coil to form a stable unit as a result of manufacturing the body coil with vacuum casting or vacuum die-casting methods.
The functioning of the RF antenna or the shielding thereof with the RF shield can be greatly improved by manufacturing the body coil with vacuum casting or vacuum die-casting methods because functional elements of the RF antenna of the body coil can be cast with the body coil on an arbitrary radius. Capacitances of the RF antenna likewise can be cast in as fixed components or overlapping structures and thus are optimally protected against external arcing or other disturbing effects.
Cooling elements also can be cast into the body coil by manufacturing the body coil with vacuum casting or vacuum die-casting methods, these cooling elements having a far greater efficiency than cooling elements that are applied onto the surface of the body coil.
Material having a low dielectric constant can be locally introduced into the mold in carrying tube regions with a high electrical field intensity when manufacturing the body coil with vacuum casting or vacuum die-casting methods and can be subsequently cast with the tube. As a result dielectric losses are kept small and the capacitive coupling of the RF field to the patient is improved.
Mechanically weak regions of the surface of the body coil can be reinforced by introducing a reinforcement into the mold before the casting.
Rovings and/or woven mats and/or pre-pregs are inventively employed for reinforcement.
The casting material can be suitably optimized by adding fillers.